We moved into our home a year ago that was built in the 70's. Whenever we turn certain ceiling fans or lights on or when the A/C engages, there is a fairly loud popping noise that comes from inside the wall. We know it's related to these switches because we can cause the noise to reoccur by turning these off and on. We did not notice it until a few months ago. What could be causing this and is it dangerous? Should I have an electrician come out?

Yes, it's probably a loose connection and vibration is causing the contact to open and close. When it sparks, it pops. Sparks have the potentential of causing a fire and intermittant connections can overheat connections. It's probably contained inside a junction box in the switch or ceiling fan.

Have it looked at.

Question, do you know if the house is wired with aluminum wiring?

Is the "Popping" noise coming from the area of the wall switch? What is on the other side of the wall?

Aluminum wiring has some inherent problems and it meeds to be periodicaly checked to make sure connections are trouble free.

Long shot, but it could be X10 or a relay.

More likely, since it occurs on several switches, it could be a neutral issue. See if you can pinpoint the source of the noise. Do any of the lights dim or flicker when items are switched on and/or when you hear the noise?